1. Field of the Invention
The present invention relates to equipment for injection blow molding hollow thermoplastic containers, and in particular to molding equipment operated under automatic computer control.
2. Description of the Related Art
Machines for injection blow molding (IBM) of hollow thermoplastic containers are known. In an IBM machine, a preform or parison is injection molded into a mold, around a core rod. The injection mold is opened and the core rod and the parison are transferred to a blow molding station. The parison is then blown into the shape of the final article inside the blow mold. The blow mold is opened and the blown article is advanced to a station where it is stripped off the core rod. After injection of the plastic to form a parison, but before blowing the parison into a finished article the parison must usually be conditioned for at least a short period of time in order to allow equilibration of the plastic to a proper temperature to enable the blowing of the article.
Plastic material may be fed through a dryer to remove moisture and is then placed in a hopper. The material falls, under its own weight, into a barrel with a reciprocating feed screw called an injection screw. The injection screw rotates in the barrel, advancing the material. The material melts due to the shear effect from the rotating barrel, and from the temperature in barrel. At the forward end of the injection screw is a ram adapted to inject the melted material (also referred to as the melt) into the parison mold. The forward and backward reciprocating motion of the injection screw causes the injection of a predetermined amount of plastic into a parison mold.
In a widely used type of injection blow molding machine, the work pieces (parisons) are first injection molded then blown and removed at (typically 3 or 4) successive work stations. Transfer between work stations is accomplished by a turret which rotates about a vertical axis. Work stations are radially disposed about the axis. The turret has a number of sides equal to the number of work stations. Core rods radially project from each of the turret sides towards the respectively facing work stations. A different operation is performed at each work station, and parisons or finished articles are moved from one station to another when all work has been completed in each cycle.
In order to provide closed loop control of the operation of the IBM machine, the machine is equipped with a plurality of sensors and at least one process controller. The process controller(s) determines the sequence of activities performed to construct a container, temperature setpoints, data acquisition and injection parameters. The process controller(s) also collects data from the sensors and employs predetermined criteria to apply corrective adjustments to the process. For example, one of the most widely used process controllers in injection blow molding systems is the proportional integral derivative (PID) controller. The PID controller, as its name implies, applies a gain to its input signal which has three components: the first component is proportional to the input signal, the second component varies with the integral of the input signal, and the third component varies with the derivative of the input signal. The input signal to the controller depends on predetermined setpoint values used for the individual processes, and the deviations from the setpoints.
The process controller is only effective when it is properly commanded to apply appropriate adjustments to the process. Effective use of the process controller depends on the operator's ability to assess the "quality" of the process for a particular controller configuration. The operator must assimilate large quantities of trend data, often within a short period of time. The operator needs a clear understanding of how the machine is performing and which parameters are not within desired limits before he or she can adjust the parameters or limits.
Software for operating the process controller has left much to be desired in several areas, particularly in systems which are used to operate several IBM machines with respective process controllers. Typically, data are stored locally in a respective processor coupled to each process controller. To monitor the data from a central station, the requested data must be fetched from the storage at the machine where the data were collected, and the data are then transmitted over a network to the central station. This causes erratic traffic loading on the network. To accommodate the traffic loads, a separate fileserver is often required.
Another problem in previous systems is the ability to respond appropriately when one of the processors reboots or encounters a power failure. Typically, the software controlling the process restarts automatically, but does not re-establish a network connection. As a result, subsequent attempts to access any services over the network (e.g., to print data from an IBM machine on a printer at the central station) may fail.